Method and apparatus for preparing semiconductor wafers for measurement

ABSTRACT

A wafer-cleaning module is disclosed for removing contaminants from a semiconductor wafer prior to measurement in a metrology tool. The cleaning module includes a heating chamber including a heater plate for heating the wafer by conduction. A separate cooling chamber is provided to cool the wafer. The system is controlled by a processor so the heating cycle, cooling cycle and the time periods between these cycles and the measurement cycle are uniform for all wafers.

TECHNICAL FIELD

This application relates to optical inspection equipment used toevaluate parameters of thin films on semiconductor wafers. The subjectinvention includes a cleaning module for reducing contaminants on thesurface of the wafer prior to measurement to improve the accuracy andrepeatability of the optical measurements. In the preferred embodiment,the cleaning module includes separate heating and cooling chambers forprocessing the wafer prior to measurement in the metrology tool.

BACKGROUND OF THE INVENTION

For many years, devices have existed for evaluating parameters of asemiconductor wafer at various stages during fabrication. There is astrong need in the industry to evaluate the parameters of multiple-layerthin film stacks on wafers using non-contact optical metrology tools. Inthese devices, a probe beam of radiation is directed to reflect off thesample and changes in the reflected probe beam are monitored to evaluatethe sample.

One class of prior measurement devices relied on optical interferenceeffects created between the layers on the sample or the layer and thesubstrate. In these devices, changes in intensity of the reflected probebeam caused by these interference effects are monitored to evaluate thesample. In many applications, the probe beam is generated by a broadband light source and such devices are generally known asspectrophotometers.

In another class of instruments, the change in polarization state of thereflected probe beam is monitored. These devices are known asellipsometers.

As thin films and thin film stacks have become more numerous andcomplex, the industry has begun developing composite measurement toolsthat have multiple measurement modules within a single device. One suchtool is offered by the assignee herein under the name Opti-Probe 5240.This device includes a number of measurement modules including a broadband spectrophotometer and a single wavelength, off-axis ellipsometer.The device also includes a broadband rotating compensator ellipsometeras well as a pair of simultaneous multiple angle of incidencemeasurement modules. The overall structure of this device is describedin PCT WO 99/02970, published Jan. 21, 1999, incorporated herein byreference. The Opti-Probe device is capable of deriving informationabout ultra-thin films and thin film stacks with a high degree ofprecision.

There is a trend in the semiconductor industry to utilize very thinlayers. For example, today, gate dielectrics can have a thickness lessthan 20 Å. It is anticipated that even thinner layers will be used.There is a need to measure the thickness of these very thin layers witha precision and repeatability to better than 0.1 Å. While the Opti-Probedevice is capable of making such measurements with the necessaryprecision, problems have arisen with respect to repeatability,especially with ultra thin films. Repeatability means that if the samemeasurement is made at two different times, the same result for layerthickness will be produced.

After considerable investigation, it has been determined that variationsin measurements over time are strongly affected by atmosphericconditions such as temperature, humidity and exposure time to the air.For example, the measured layer thickness could be considerably higherwhen the humidity is relatively high. In addition, the thickness of thelayer can be effected by the growth of a contaminant layer, even in socalled “clean room” environments. In fact, it is known that a clean roomcan contain a wide variety of contaminants including plastics,lubricants, solvents, etc. The variation in measurement due solely toatmospheric conditions can be on the order of 0.1 Å which substantiallyreduces the likelihood of making repeatable measurements with aprecision of 0.1 Å. In order to improve the repeatability of themeasurements results, it would be desirable to remove the contaminantlayer prior to measurement.

There are many types of wafer cleaning procedures used in asemiconductor fabrication facility. However, any cleaning procedureswhich require contact with the wafer, such as cleaning solutions, wouldnot be desirable at this stage of fabrication since it can damage orcontaminate the gate dielectric or the wafer. Additionally, mostchemical cleaning processes require a drying cycle during which time anew hydrocarbon contamination layer could reform.

One suitable type of wafer cleaning system is described in our copendingapplication Ser. No. 09/294,869, filed Apr. 20, 1999, and incorporatedherein by reference. One embodiment of the system described in thelatter patent application includes a microwave generator for excitingwater molecules in order to drive off contaminants. Another approachdescribed in the latter application was the use of a radiant heatingsource to drive off contaminants. Various additional combinationsincluding microwave and radiant excitation along with UV radiation or astream of frozen carbon dioxide pellets were suggested.

Another wafer cleaning system is described in PCT Application Ser. No.WO 99/35677 published Jul. 15, 1999. The device disclosed in thisapplication relies primarily on radiant heating using tungsten halogenquartz lamps. An important aspect of the device in the latterapplication is the presence of high-energy light wavelengths forbreaking bonds in the contaminant layer. The wafer cleaner described inthis PCT application has a single chamber. Cooling can be achievedthrough the use of a water-cooled bottom reflector in the chamber.

After considerable experimentation, it has been determined that theprincipal mechanism for removing contaminants in the approachesdescribed above relates directly to an increase in the temperature ofthe wafer. Although microwave excitation and radiant light exposure bothfunction to increase the temperature of the wafer, the latter twoapproaches are not the most efficient method of raising the temperatureof the wafer. Therefore, it is believed that the best approach forpreparing a wafer for measurement is to heat the wafer directly, byconduction.

Direct conductive heating has many advantages. For example, directconductive heating can raise the temperature of the wafer to the desiredtemperature much faster than with either microwave or radiant energyexposure given the same amount of input energy. In addition, directconductive heating can produce a more uniform and repeatable temperaturerise in the wafer without complex equipment design.

Further experimentation also revealed that optimal results can only beachieved if the process is carefully controlled. Careful controlincludes heating each wafer to the same temperature, subjecting eachwafer to the same cooling cycle and insuring that the time between theend of the cooling cycle and the beginning of the measurement cycle inthe metrology tool is the same for all wafers.

Accordingly it is an object of the subject invention to provide animproved wafer-cleaning module which can accurately and repeatablyremove contaminants from a wafer prior to measurement in a metrologytool.

SUMMARY OF THE INVENTION

A wafer-cleaning module is disclosed which includes a heating stationhaving a planar heater element for heating the wafer by conduction. Inthe preferred embodiment, the heater element is a plate formed from adielectric material such as alumina. The plate has a thin layer of aresistive material attached or deposited on the underside thereof. Anelectrical current applied to the resistive layer creates heat whichdiffuses evenly through the plate. A set of lift pins can be provided toraise and lower the wafer onto the plate. The lift pins are provided topermit a robotic arm to more easily load and remove the wafer from theheating station.

The wafer-cleaning module further includes a separate, thermallyisolated cooling station. The cooling station includes a planar heatsink surface which can be air or water-cooled. Having separate heatingand cooling stations allows the wafer to be cooled faster and moreefficiently than if the cooling is performed within the heating station.

In accordance with the subject invention, the cleaning module is placedunder the control of a processor. In order to achieve repeatability ofthe measurement of the wafer, the heating and cooling steps must be thesame for each of the wafers being tested. To the extent possible, eachwafer should be heated to roughly the same temperature and held at thattemperature for roughly the same period of time. Each wafer should besubjected to the same cooling cycle. In addition, the time periodbetween the end of the cooling cycle and the initiation of themeasurement cycle should also be the same for each wafer.

In the preferred embodiment, a robotic arm loads the wafer into theheating station (chamber). The processor controls the heating chamberbased on both time and feedback from temperature sensors in the chamber.When the heating cycle is complete, the robotic arm will transfer of thewafer from the heating chamber to the cooling station (chamber). Afterthe cooling cycle is complete, the robotic arm will transfer the waferto the metrology tool for measurement. As noted above, each of thevarious cycles and periods between cycles should be the same for eachwafer.

Further objects and advantages of the subject invention will becomeapparent from the following detailed description taken in conjunctionwith the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective you illustrating the cleaning module of thesubject invention mounted in conjunction with a metrology tool.

FIG. 2 is an enlarged view of the wafer-handling portion of the cleaningmodule of the subject invention.

FIG. 3 is that exploded perspective view of the cleaning module of thesubject invention.

FIG. 4 is a perspective view of the cleaning module of the subjectinvention with the shroud removed.

In FIG. 5 is a perspective view, partially in section, of the cleaningmodule of FIG. 4.

FIG. 6 is a cross-sectional view of the cleaning module showing the liftpins in the heating station in the raised position.

FIG. 7 is a cross-sectional view similar to FIG. 5 showing the lift pinsin the lower or retracted position.

FIG. 8 is a graph illustrating thickness measurements and including acomparison between a wafer which has been subjected to the cleaningprocess throughout repeated measurements and another wafer where somemeasurements are taken without subjecting the wafer to a cleaningprocess.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of the cleaning module 10 shown integratedwith a metrology tool 12. The metrology tool 12 includes one or moremeasurement modules of the type described in PCT WO 99/02970 andmarketed by the assignee under the name Opti-Probe 5240. The metrologytool includes an opening 14 for receiving wafers 16 are held in acassette (not shown) loaded on a cassette station 18 prior to beingplaced into the heating module 10. A processor controlled robotic arm 20is provided for taking wafers from the cassette and inserting them intothe cleaning module 10 and thereafter into the metrology tool 12.

In FIGS. 1 and 2, the robotic arm 20 shown in solid lines is orientedtowards the wafer stack. The free end of the arm is also shown inphantom line in these figures and illustrates the insertion of a waferinto the cleaning module.

Cleaning module 10 includes a lower heating chamber 30 having anentrance slot 32 and an upper cooling chamber 36 with an entrance slot38. Alternatively, the heating chamber could be located above thecooling chamber. FIGS. 3, 4 and 5 provide a more detailed view of theinternal elements of the cleaning module 10. As seen therein, theheating station 30 includes a planar heater element 40. In the preferredembodiment, the heater element or plate 40 is formed from a dielectricmaterial such as alumina. A resistive material is connected to thebottom surface of the plate 40. In the preferred embodiment, theresistive material is deposited on the lower surface of the plate. Thedeposited layer is thin, compact and can rapidly achieve ahigh-temperature when electric current is supplied thereto. Preferably,the diameter of the heater element is larger than the diameter of thewafer 16.

The upper surface of heater element 40 includes a pattern of grooves 46.The grooves 46 are connectable to a source of vacuum via a hole (notshown) drilled from the bottom side of the plate up to and intocommunication with the groove 46. Application of a vacuum to the groovedraws the wafer into close physical contact with the plate 40 tomaximize heat transfer and control uniformity of the heating.

To facilitate the loading and unloading of a wafer into the heatingchamber, a plurality of movable lift pins 50 are provided. As seen inFIGS. 5 and 6, lift pins 50 are slidable within channels 52 provided inplate 40. When a wafer is being loaded into the heating chamber, thelift pins 50 are positioned in the upper orientation. In this position,the robotic arm 20 can load the wafer into the heating chamber throughslot 32 and lower the wafer onto the top of pins 50 (See FIG. 6). Oncethe wafer is in place, the robotic arm can be lowered and withdrawn fromthe heating chamber. At this point, pins 50 are lowered so that thewafer is in contact with the plate 40 (See FIG. 7). The pins are carriedon a fixture 56 which is connected to a lift mechanism 58 via anextended arm 60.

It has been found desirable to run the vacuum system as the wafer isbeing lowered onto the plate by the pins. By this approach, the waferwill be firmly grabbed by the plate and will not slide around on a thinlayer of air that could otherwise be compressed under the wafer as itdrops onto the planar surface of the plate. To facilitate handling, thefree end 62 (FIG. 2) of the robotic arm has a vacuum port for drawingthe wafer into tight contact with the arm while the wafer is being movedbetween stations.

Heating station 30 is provided with a purge gas system. A gas inlet andgas outlet (not shown) are provided on opposite sides of the chamber.The purge gas is preferably an inert species such as nitrogen.Circulation of the purge gas can facilitate removal of the contaminantswhich have been freed from the surface of the wafer during heating.Various temperature sensors can also be provided to permit automaticmonitoring of the cleaning process.

The heating chamber is preferably water cooled to limit the amount ofheat which can escape from the heating chamber into the environment. Asbest seen in FIG. 5, a heat exchanger 70 is located in the bottomportion of the heating chamber. The bottom surface of heat exchangerincludes a plurality of fins 72 through which a cooling fluid is passed.

The upper portion of the cleaning module 10 includes a cooling chamber36. Mounted within cooling chamber 36 is a metal plate 80. Metal plate80 is in thermal contact with a heat exchanger 82. Heat exchanger 82 hasa plurality of fins 84 through which a cooling fluid is passed.

Similar to heating plate 40, cooling plate 80 includes a pattern ofgrooves 88 which are connectable to a source of vacuum. When activated,the vacuum will pull the wafer into close contact with the surface ofplate 80 to maximize heat flow from the wafer to the cooling plate.Unlike heating plate 40, cooling plate 80 includes a slot 90 configuredto receive the end of robotic arm 20. The slot allows the arm to moveinto the plate in coplanar fashion so that there is no need to employthe lift pin structure used in the heating station. A similar slot isnot provided in the heating plate since such a slot would createunacceptable thermal variation in the heating of a wafer.

It should be noted that in the preferred embodiment, the heating andcooling stations are spatially separate and thermally isolated from eachother but are mounted within the same external housing. In principle,the two chambers could be physically separated, however it is believedthe preferred embodiment is more efficient since the two chambers areclose together minimizing wafer transfer time. In addition, bothchambers can share the same cooling water. In the illustratedembodiment, the cooling water is first pushed through the low heat dutyof the cooling chamber followed by the high heat duty of the heatingchamber.

In one important aspect of the subject invention, the entire cleaningmodule and its operating cycle are under the control of a computerprocessor. These control functions include both the basic operation ofthe heating and cooling chambers and, more importantly, control of theinteraction between the heating and cooling cycles, wafer transfer timesand subsequent measurement process. Accurate control of the system withone or more processors leads to more repeatable results. Under thisapproach, each wafer to be measured will be subjected to similar heatingand cooling cycles. In addition, the time period between the coolingcycle and the beginning of the measurement cycle can be made the samefor each wafer.

Having described the basic elements of the cleaning module 10 of thesubject invention, a typical operating cycle will now be described. Inuse, a cassette holding a plurality of wafers will be placed on thecassette station 18. Robotic arm 20 will pull the first of the wafersout of the cassette and load it through opening 32 into the heatingstation 30. The wafer will be loaded onto the upper surface of pins 50.The robotic arm will then drop down away from the wafer and be retractedfrom the heating station. A vacuum will be applied to the grooves 46 andthen pins 40 will retract from the upper wafer load position as shown inFIG. 6 to the lower wafer heating position as shown in FIG. 7.

Preferably, the heater element will already be elevated to the desiredtemperature. The desired temperature will vary based on the type ofsample and nature of contaminant. The temperature should be high enoughto drive off the contaminants but low enough so that the electricaldevices on the wafer will not be damaged. In some initial experiments,it has been found that a temperature of 325 degrees C can be used.Various sensors are provided within the chamber to monitor itstemperature. In the event that the temperature rises to an unacceptablelevel, the sensors can signal the processor to shut down the heatingstation before the wafer is damaged.

Because the wafer is in close contact with the heating plate, the waferwill reach the temperature of the plate in just a few seconds. The waferwill be allowed to remain on the heated plate for a predetermined timeperiod. In practice, it has been found that a period of about 30 secondsis sufficient to drive off the contaminant layer. Note that since thewafer is heated to the desired temperature quickly (as compared to otherapproaches such as with radiant or microwave heating), the total heatingtime can be minimized, increasing throughput.

Once the predetermined time period has elapsed, the vacuum is turned offand the wafer will be raised off of the plate by pins 50 and removedfrom the heating station by robotic arm 20. The wafer will immediatelybe placed into the cooling station through opening 38 by robotic arm 20.Preferably, the vacuum is activated before the wafer is loaded on plate80 so the wafer will be grabbed by the plate and not slide around. Asnoted above, the robotic arm will be received in slot 90 of plate 80allowing the vacuum to grab the wafer. Once the wafer is grabbed by thevacuum, the robotic arm can be removed.

As noted above, plate 80 is water-cooled. In practice, it has been foundthat the wafer needs to remain on the plate 80 for only about 10 secondsin order to lower the temperature of the wafer to the ambienttemperature

Once the wafer has reached room temperature, it is removed from thecooling station by the robotic arm. Preferably, the wafer is loaded intothe metrology tool very soon after the cooling cycle has been completed.As noted above, the contaminant layer begins to regrow on the waferimmediately after the heating cycle has been completed. Therefore thetime period between the end of the heating cycle and the beginning ofthe cooling cycle, as well as the end of the cooling cycle and thebeginning of the measurement cycle, should be minimized.

In accordance with the subject invention, the processor will control theprocessing of each of the subsequent wafers in the same manner as thefirst wafer. The time at which the next wafer is placed into the heatingchamber will depend upon the length of the measurement cycle. Forexample, if the measurement cycle is quite long, the processor will waituntil near the end of the measurement cycle before beginning to load thenext wafer into the heating chamber. Conversely, if the measurementcycle is quite short, the processor would able to begin the heatingcycle near the beginning of the measurement cycle.

FIG. 8 is a graph illustrating multiple measurements over time of twowafers. Both wafers were pre-measured two times at the start of theexperiment. Each wafer was then re-measured every twenty minutes. Thewafer from slot 10 (open boxes in graph) was subjected to repeatedheating and cooling cycles before every measurement. As can be seen, thelayer thickness did not vary significantly over all the subsequentmeasurement cycles. The wafer from slot 5 (closed boxes in graph) wassubjected to cleaning cycles before each of the first sevenmeasurements. This wafer was then measured eight more times withoutbeing subjected to any cleaning cycles. As can be seen, the thickness ofthe contaminant layer increased between each of the measurements wherethe wafer was not subjected to pre-cleaning. In fact, the thickness ofthe contaminant layer increased by about 1.3 Å over a period of lessthan three hours.

The wafer in slot 5 was then subjected to cleaning cycles before each ofthe remaining measurements. As can be seen, the cleaning cyclesimmediately reduced the thickness of contaminant layer back to theoriginal minimal levels. This experiment demonstrates that all real-timefilm growth from the wafer can be removed in a single pass through thecleaning cycle. The thickness measurements in FIG. 8 are mean valuesobtained at 17 coordinates distributed across the wafer to achieve ameaningful sample of the representative oxide layer thickness value.

While the subject invention has been described with reference to apreferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

What is claimed:
 1. A method of preparing semiconductor wafers formeasurement in a metrology tool comprising the steps of: a) heating thewafer by conduction in a heating chamber for a predetermined time periodto remove contaminants from the wafer surface; b) cooling the wafer byconduction in a cooling chamber separate from the heating chamber for apredetermined time; and c) measuring characteristics of the wafer withthe metrology tool a predetermined time after the cooling step iscompleted.
 2. A method as recited in claim 1, wherein steps (a) (b) and(c) are repeated for a plurality of wafers.
 3. A method of preparingsemiconductor wafers for measurement in a metrology tool comprising thesteps of: a) loading a wafer onto a planar heater element located in aheating chamber; b) heating the wafer by conduction for a predeterminedamount of time to remove contaminants from the wafer surface; c)removing the wafer from the heating chamber; d) loading the wafer onto aplanar cooling element located in a cooling chamber thermally isolatedfrom the heating chamber; e) cooling the wafer for a predeterminedamount of time; f) removing the wafer from the cooling chamber andloading the wafer in the metrology tool; g) measuring characteristics ofthe wafer with the metrology tool.
 4. A method as recited in claim 3,wherein steps (a) through (g) are repeated for a plurality of wafers. 5.A method as recited in claim 4, wherein the predetermined heating timeis the same for the plurality of wafers.
 6. A method as recited in claim4, wherein the predetermined cooling time is the same for the pluralityof wafers.
 7. A method as recited in claim 4, wherein the time betweenthe end of cooling step and the beginning of the measurement step is thesame for the plurality of wafers.
 8. A method as recited in claim 4,wherein the time between the end of the heating step and the beginningof the cooling step is the same for the plurality of wafers.
 9. A methodas recited in claim 4, wherein: the predetermined heating time is thesame for the plurality of wafers; the predetermined cooling time is thesame for the plurality of wafers; the time between the end of theheating step and the beginning of the cooling step is the same for theplurality of wafer; and the time between the end of cooling step and thebeginning of the measurement step is the same for the plurality ofwafers.
 10. A method of preparing semiconductor wafers for measurementin a metrology tool comprising the steps of: a) heating the wafer in aheating chamber to remove contaminants from the wafer surface; and b)cooling the wafer in a separate cooling chamber thermally isolated fromthe heating chamber.
 11. A method as recited in claim 10 furtherincluding the step of measuring characteristics of the wafer with themetrology tool after the cooling step is completed.